The present invention is directed to an improved method and device to be used in testing the accuracy of positive displacement diaphragm type gas meters and in particular, natural gas meters. A key aspect of the present invention is the provision of a magnetic sensor for accurately metering gas flow through a diaphragm gas metering device.
In order to test or prove a positive displacement diaphragm meter, a test gas, normally air, is passed from an accepted standard (Bell Prover, Sonic Nozzle, Master Meter, etc.) through the test meter. The volume of air passed by the standard is compared to the volume registered by the test meter (as read on the meter index). This relationship is known as the "meter accuracy" or its reciprocal, the "meter proof".
There are two primary prior art methods of getting the start and end points of a meter accuracy test. In the first or conventional proving method, a photo-electric device is used to determine when a preset number of revolutions of the meter index proving dial have occurred. When the proving dial pointer first breaks the photo-electric light beam, the volume of the standard is noted as the initial volume. The final volume is noted after the beam has been broken the preset number of times. The start volume is subtracted from the end volume and then compared to the volume indicated by the test meter index, thereby providing the accuracy of the test meter.
The second method is grounded in the phenomenon that individual diaphragm meters have their own unique pressure "signatures" (See, e.g. U.S. Pat. No. 3,937,048). Simply stated, diaphragm meters have one or more pressure peaks and troughs during each cycle of the diaphragm, and these peaks and troughs tend to periodically repeat themselves. The pressure signature method chooses one of the pressure peaks and attempts to detect this peak during subsequent cycles (known in the industry as "tangent revolutions"). In a typical test, the pressure peak which is chosen starts the test, and the test continues for a preset number of pressure peaks. The accuracy is calculated as in the first method.
There are several problems associated with the prior art methods of gas meter testing. First, both prior art methods are more time consuming. The photo-electric method normally passes two cubic feet of test gas through the meter under test, and requires approximately 4 minutes to complete a test. The pressure signature method, while passing a fraction of a cubic foot of gas, requires approximately 85 seconds to perform the same test.
The pressure signature method has the additional problem of requiring the repetitive identification of one of the many peaks which occur within each tangent revolution. This is sometimes problematic because the amplitude of the peaks can vary between tangent revolutions, thereby leading to improper readings.
Moreover, in the pressure signature method, if an incorrect peak is sensed, there is no provision for warning the user. Further, because the pressure signature method relies upon the identification of a specific peak, several tangent revolutions may have to be performed in order to identify an appropriate starting point.
Finally, the pressure signature method requires expensive equipment including an extremely sensitive pressure transducer, a hybrid circuit board (to condition the pressure signal into a single triggering pulse, for each tangent revolution) and a DC power supply.
It would be desirable to have a gas meter test device and method which eliminates the problems associated with the conventional and pressure signature methods.
It would be particularly desirable to have a novel magnetic sensor which can be utilized in conjunction with the testing or proving of diaphragm meters and which can be placed in proximity to the opaque metallic housing of a gas meter to be tested.
It is therefore an object of the present invention to provide a gas meter tester which can quickly prove or test a positive displacement diaphragm meter on fewer tangent cycles of the meter. The present invention can prove a meter on three tangent revolutions with a commensurate time savings over the pressure signature proving method and the conventional proving method.
It is a further object of the present invention to provide a proving or testing device which is triggered by the movement of the diaphragm connecting arm itself, and therefore does not require the repetitive identification of a single pressure peak.
It is a further object of the present invention to provide a test device and method which is compatible for use with a computer for storing signals corresponding to the volume of gases passed between cycles.
It is still a further object of the present invention to provide a magnetic sensor for a gas meter proof testing device, which can be placed outside the opaque housing of a test meter and which can magnetically monitor the direct movement of the diaphragm arm.
It is yet a further object of the present invention to provide a test device and method which can be performed utilizing inexpensive equipment including a magnetic position sensor, a PC compatible parallel port, a D/A Converter and a voltage source supplied by the computer.